1. Field of the Invention
The present invention relates to a variable displacement compressor. More particularly, it relates to an improvement of a variable angle wobble plate type compressor.
2. Description of the Related Art
A typical variable angle wobble plate type compressor adapted to be used in a vehicle airconditioning compressor is disclosed in U.S. Pat. No. 4,428,718. In the compressor, a control arrangement, having a valve mechanism biased by bellows and springs and responsive to both suction and discharge pressures, is provided to control the compressor crankcase pressure with respect to the suction pressure so as to change a wobble angle of a rotary drive plate on a drive shaft, thereby increasing or decreasing the compressor displacement, i.e., the discharge flow rate. When a cooling load, i.e., an air conditioning capacity demand, is lowered or when the compressor rotating speed is increased, a decrease in the suction pressure occurs. Thus, the bellows of the control arrangement are expanded, due to a change in the balance between the suction and atmospheric pressures, to operate the valve mechanism of the control arrangement. As a result, communication between the compressor suction chamber and crankcase is reduced, and alternately, communication between the compressor discharge chamber and crankcase is expanded so that the crankcase pressure is increased, thereby increasing a pressure differential between the suction pressure and the crankcase pressure. That is, a pressure acting behind the pistons of the compressor is increased resulting in a decrease in the length of stroke of the pistons. As a result, the suction pressure to the compressor is recovered, and the compressor displacement is decreased.
However, in the control arrangement of the above-mentioned typical conventional compressor, the valve mechanism is constructed so as to open and close both the communicating passageways between the suction chamber and crankcase and between the discharge chamber and crankcase. This makes the construction of the valve mechanism complicated. Also, when the compressor is rotated at a high speed and a high load is applied to the seal arrangement of the drive shaft, the crankcase pressure is increased. Therefore, the sliding portion of the seal arrangement of the drive shaft is subjected to a high surface pressure. As a result, the sealing performance as well as the physical durability of the seal mechanism must be lowered. The lowering of the sealing performance also occurs when the vehicle engine for driving the compressor suddenly speeds up. This is because, when the vehicle engine speed is rapidly accelerated, the suction pressure to the compressor is lowered and the above-mentioned control arrangement is operated so as to increase the crankcase pressure.